Mac Diarmada was born John MacDermott in Corranmore, close to Kiltyclogher in County Leitrim, an area where the landscape was marked by reminders of poverty and oppression. His father Donald McDermott was a member of the IRB and a friend of John Daly. Surrounding Mac Diarmada in rural Leitrim, there were signs of Irish history throughout the area. There was an ancient sweat-house, Mass rocks from the penal times and the persecutions of the 17th and 18th centuries, and deserted abodes as an aftermath of the hunger of the 1840s. He was educated by the Irish Christian Brothers. In 1908 he moved to Dublin, by which time he already had a long involvement in several Irish separatist and cultural organisations, including Sinn Féin, the Irish Republican Brotherhood, the Ancient Order of Hibernians and the Gaelic League. He was soon promoted to the Supreme Council of the IRB and eventually elected secretary. He originally refused to join the IRB as it was condemned by the Catholic church but Bulmer Hobson convinced him otherwise. In 1910 he became manager of the radical newspaper Irish Freedom, which he founded along with Bulmer Hobson and Denis McCullough. He also became a national organiser for the IRB, and was taken under the wing of veteran FenianTom Clarke. Indeed, over the year the two became nearly inseparable. Shortly thereafter Mac Diarmada was stricken with polio and forced to walk with a cane. In November 1913 Mac Diarmada was one of the original members of the Irish Volunteers, and continued to work to bring that organisation under IRB control. In May 1915 Mac Diarmada was arrested in Tuam, County Galway, under the Defence of the Realm Act 1914 for giving a speech against enlisting into the British Army during the First World War.
Following his release in September 1915, he joined the secret Military Committee of the IRB, which was responsible for planning the rising. Indeed, Mac Diarmada and Clarke were the people most responsible for it. Mac Diarmada, like the rest of the signatories of the proclamation believed a "blood sacrifice" was necessary. In 1914 he said "the Irish patriotic spirit will die forever unless a blood sacrifice is made in the next few years.” Due to his disability, Mac Diarmada took little part in the fighting of Easter week, but was stationed at the headquarters in the General Post Office, as one of the Provisional Republican Government. Following the surrender, he nearly escaped execution by blending in with the large body of prisoners. He was eventually recognised by Daniel Hoey of G Division. Following a court-martial on 9 May, Mac Diarmada was executed by firing squad on 12 May at the age of 33. In September 1919 Hoey was shot dead by Michael Collins's Squad. Likewise, the British Officer Lee-Wilson, who ordered Mac Diarmada be shot, rather than imprisoned, was also killed in Cork on Collins's order during the Irish War of Independence. Mac Diarmada had been in regular correspondence with Nell Ryan. In his final letter he wrote: "Miss Ryan, she who in all probability, had I lived, would have been my wife". "Min" Josephine Ryan and her sister, Phyllis, had been couriers to the GPO. They also visited Kilmainham Gaol, before his execution, and managed to evade arrest. Min, a founder of Cumann na mBan, managed to escape from Ireland to America; she later married Richard Mulcahy. Before his execution, Mac Diarmada wrote, "I feel happiness the like of which I have never experienced. I die that the Irish nation might live!”
